1. Field of the Invention
The present invention relates to modelocked fiber lasers for producing femtosecond and picosecond pulses, and particularly to modelocked fiber lasers employing double-clad fibers pumped with diode laser arrays.
2. Description of the Related Art
Modelocked fiber lasers are generally considered ideal candidates for field-suitable compact sources of both femtosecond and picosecond pulses. However, the eventual commercial success of any ultrafast technology based on fiber lasers depends heavily on the availability of simple laser designs that can compete in cost and complexity with alternative solutions. To this end, passively modelocked fiber lasers are particularly attractive, as they do not require expensive modulators for their operation.
Since passively modelocked fiber lasers were first reported by M. E. Fermann, M. Hofer, F. Haberl, A. J. Schmidt and L. Turi in "Additive-pulse-compression mode locking of a neodymium fiber laser", Optics Letters, Vol. 16, No. 4 (1991), two systems have been developed which hold promise as commercially viable passively modelocked fiber lasers for the production of femtosecond pulses. See, M. E. Fermann, L. M. Yang, M. L. Stock and M. J. Andrejco, "Environmentally stable Kerr-type mode-locked erbium fiber laser producing 360-fs pulses", Optics Letters, Vol. 19, No. 1 (1994), hereinafter denoted as system 1; and E. A. DeSouza et al., "Saturable Absorber Modelocked Polarisation Maintaining Erbium-doped Fibre Laser", Electronics Letters, Vol. 29, No. 5 (1993), hereinafter denoted as system 2.
Two different systems have been developed which hold promise as commercially viable picosecond pulse lasers. See, M. E. Fermann, K. Sugden and I. Bennion, "High-power soliton fiber laser based on pulse width control with chirped fiber Bragg gratings", Optics Letters, Vol 20, No. 2 (1995), hereinafter denoted as system 3; and B. C. Barnett et al., "High-power erbium-doped fiber laser mode locked by a semiconductor saturable absorber", Optics Letters, Vol. 20, No. 5 (1995), hereinafter denoted as system 4.
Systems 2 and 4 rely on a saturable absorber for initiation of modelocking and also for steady-state pulse shaping. In contrast, systems 1 and 3 use a saturable absorber only for the initiation of modelocking, and obtain steady-state pulse shaping by nonlinear polarization evolution in the fiber. Advantageously, systems 1 and 3 additionally include a compensation scheme based on two Faraday rotators, which strongly suppresses linear and nonlinear polarization drifts.
In the picosecond regime, system 3 allows the formation of a wide range of pulse-widths simply by changing the intra-cavity dispersion with a chirped fiber Bragg grating (CFBG). See, e.g., M. C. Farries, K. Sugden, D. C. J. Reid, I. Bennion, A. Malony and M. J. Goodwin in "Very broad reflection bandwidth (44 nm) chirped fibre gratings and narrow bandpass filters produced by the use of an amplitude mask", Electronics Letters, Vol. 30, No. 11 (1994).
While these systems work perfectly well in the laboratory, from a commercial point of view, these systems are still only of limited interest, since these systems require expensive pump sources such as ion or solid-state lasers, master-oscillator power amplifier laser diodes or even high-power pig-tailed single-mode diode lasers. In contrast, passively modelocked solid state lasers that produce similar pulse widths may routinely be pumped with low-cost, broad-area, multi-stripe diode laser arrays, as disclosed by K. J. Weingarten, U. Keller, T. H. Chiu and J. F. Ferguson in "Passively mode-locked diode-pumped solid-state lasers that use an antiresonant Fabry-Perot saturable absorber", Optics Letters, Vol. 18, No. 8 (1993) and D. Kopf, K. J. Weingarten, L. R. Brovelli, M. Kamp and U. Keller in "Diode-pumped 100-fs passively mode-locked Cr:LiSAF laser with an antiresonant Fabry-Perot saturable absorber", Optics Letters, Vol. 19, No. 24 (1994). The feasibility of pumping with diode laser arrays makes passively modelocked solid state lasers very attractive despite their typically significantly larger physical dimensions.
To minimize cost, modelocked fiber lasers also should employ diode laser arrays. Indeed, it has been long known that continuous wave fiber lasers may be pumped by diode laser arrays when a doubleclad structure is employed in the fiber design. See, e.g., U.S. Pat. No. 4,815,079 to Snitzer et al. According to Snitzer et al., the fiber is designed to have two claddings, wherein the outer cladding has a low refractive index and the inner cladding has a significantly higher refractive index, giving a typical numerical aperture for light capture by the inner cladding between 0.20 and 0.60. The fiber core then has an even higher refractive index and is placed inside the inner cladding, such that the core location is significantly offset from the center of the inner cladding.
Snitzer et al. alternatively disclose the inner cladding as having a nearly rectangular shape. Both of these designs ensure that any light launched into the inner cladding crosses over the fiber core as often as possible, so that the light may be efficiently absorbed when the fiber core is doped with a rare-earth gain material. The fiber core may then be designed to be single-mode, and, as a result, a single-mode laser signal output may be obtained when the fiber is placed into a resonator. Note, however, that perfectly acceptable performance from double-clad fibers having a centro-symmetric fiber structure, i.e., a fiber core placed in the center of the inner cladding, was recently demonstrated. H. Zelmer, U. Williamkowski, A. Tunnerman and H. Welling, "High-power cw neodymium-doped double-clad fiber lasers", CLEO 95, paper CMB4. Such pumping schemes were previously predicted in U.S. Pat. No. 3,808,549 to Maurer. The fiber design can then be reduced to that of a standard single-mode fiber with a low-index coating (such as silicone rubber), which, in fact, was the industry standard for fiber fabrication before the advent of acrylate coatings.
Passive modelocking was recently demonstrated in such centro-symmetric double-clad neodymium-doped fibers cladding pumped by multi-stripe diode laser arrays. M. Minden et al., in "Long-pulse coherent waveforms from a fiber laser", CLEO 95, paper CTuR2. Specifically, an unchirped narrow-bandwidth fiber grating was used to limit the bandwidth of the generated pulses and a saturable absorber was used both for pulse initiation and steady-state pulse shaping. However, no schemes for the compensation of linear and nonlinear polarization drifts in the cavity were used, and only pulses with widths of .apprxeq.500 psec and longer could be generated. Further, no means for controlling the intra-cavity dispersion and no means for suppressing cladding modes were included in that work. Therefore, in that work as to date, it has not been possible to construct cladding-pumped fiber lasers that produce femtosecond or picosecond pulses.